1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP-GMR sensor with an improved spacer layer.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically formed of Cu or Ag. One ferromagnetic layer, typically called the “reference” layer, has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer, typically called the “free” layer, has its magnetization direction free to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as current-perpendicular-to-the-plane (CPP) sensor.
CPP-GMR sensors are to be distinguished from CPP tunneling magnetoresistance (TMR) sensors. In a CPP-TMR sensor spin-dependent tunneling of electrons occurs across an insulating tunnel barrier layer (like MgO or TiO2), whereas in a CPP-GMR sensor spin-dependent scattering of conduction electrons occurs at the interfaces between the conductive spacer layer and the magnetic layers as well as in the magnetic layers themselves.
In a magnetic recording disk drive CPP-GMR read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. The magnitude of the CPP-GMR effect with standard ferromagnetic materials (such as CoFe) for the free and reference layers and nonmagnetic metals such as Cu and Ag for the spacer layer is not large enough for read heads of the next generation of disk drives. The magnitude of CPP-GMR is indicated by ΔR/R (magnetoresistance or magnetoresistive ratio) or ΔRA (resistance change-area product). The voltage output of the read sensor is given by ΔR/R×Vbias, equivalently by ΔRA×Jbias, where Vbias and Jbias are the bias voltage and bias current density to the sensor, respectively. Therefore, increasing the CPP-GMR magnitude improves the read sensor performance.
The use of oxide semiconductors for the spacer layer has been proposed as a way to increase CPP-GMR magnitude. CPP-GMR sensors with spacer layers formed of semiconductor oxides like ZnO have been described, for example in U.S. Pat. No. 7,826,180 B2; U.S. Pat. No. 8,432,645 B2; and Shimazawa et al., “CPP-GMR Film With ZnO-Based Novel Spacer for Future High-Density Magnetic Recording”, IEEE Trans Magn Vol. 46, No. 6, June 2010, 1487. These sensors show ΔR/R up to about 20% with RA values of about 200-300 mΩ·μm2. However, resistance noise becomes a dominant source of noise in CPP-GMR sensors when the RA value exceeds about 100 mΩ·μm2, which limits the value of these semiconductor oxide spacer layers. Additionally, the large variation of RA values of devices on a wafer with ZnO spacer layers has been observed, which is an obstacle to high yield wafer-scale of manufacturing of sensors with these materials as spacer layers.
What is needed are CPP-GMR sensors with high ΔR/R and acceptable RA values and that can manufactured at wafer-scale with small variations in RA values.